I have had the same thing happen on my 8330 Curve. The phone can just be sitting on the table and one minute it will make a noise for a message or call and then after i respond to a message, it wont even make a noise when then next message comes in. Just flashes red up in right hand corner. I have also had the problem of it switching my ring tone from the one i chose to the original one that the phone came with and that's when i know its about to not make any sound at all.